Okay, so these products (SoundBlaster E1 and SoundBlaster E3) are confusing because they have several roles. Both are billed as "headphone amplifiers" with battery power. These types of products are somewhat rare and niche on the whole. Probably the main reason for using the amplifier portion is if you had high impedance headphones. Creative claims to support 600 Ohm headphones with both of these models.
And this is where Creative started tossing other features in.
Both the E1 and E3 can be used as an external sound adapter for PCs and Macs. While features, such as EAX, have gone by the wayside due to modern audio APIs, there is still room for sound devices to differentiate in terms of signal-to-noise ratio (SNR), and so forth, especially when compared to some on-board solutions. Speaking of SNR, the E1 advertises 106dB while the E3, 110dB. Also, sometimes you just want another sound card and USB is convenient. Both include ASIO drivers which is especially useful, although not too uncommon, for professional recording software.
The E3 then goes off on a tangent. Its USB hookup can attach not just to PCs and Mac, but also Android and iOS mobile devices. While it also has Bluetooth for iOS 5+ and Android 3.1+, it can be used as a wired, external sound card over USB on Android 4.2+ (using USB Streaming over Android Open Accessory Protocol 2.0) and iOS 7+ (using a Lightning USB adapter). This allows users to bypass the built-in amplifiers of their smartphones and tablets without Bluetooth compression. I would be interested to see reviews of this unit compared with the 3.5mm jack quality of typical mobile devices.